Facial expression is a natural and powerful tool of communication among humans. Facial expressions are instantly processed by humans conveying a wealth of messages: a smiling face spreads happiness, and a sad face makes our heart ache. There is evidence that some emotional facial expressions are largely universal [1]}, that is, they are recognized well in different cultures and associated with similar semantic content. Recent work, however, has cast some doubt to the degree of this universality [2]}, [3]} and the degree to which facial expressions are actually a reliable signal of an internal mental state [4]}, but that does not lessen the importance of facial expressions in human-to-human communication [5]}.
